1. Technical Field
This invention relates generally to methods and apparatus for power conversion, and more particularly, to an apparatus and method for combining the energy available from multiple sources, including those with low total energy content and high power delivery capability, as well as those having high total energy content and low power delivery capability, to supply a single load, and conversely, to transfer power from a single load to multiple, varied energy sources.
2. History of Related Art
Batteries remain the most widely accepted electrical storage devices despite their volume, weight, and life-cycle usage limitations. Designs of both all-electric and hybrid vehicles rely significantly on the availability of advanced battery technology for energy storage. Since battery life depends upon the charge and discharge rates employed during operation, it can be enhanced significantly by maintaining an optimum usage profile. Of course, this also means that the life of a battery can be shortened considerably if the use requirements are such that the battery is charged and discharged rapidly.
To augment battery power, auxiliary energy systems such as ultracapacitors and flywheels are used to deliver (source) or absorb (sink) power in electric vehicle applications, for example, during rapid acceleration and deceleration. Thus, the basic capability of a battery-powered electric vehicle can be enhanced significantly by supplying vehicle peak-power requirements using such auxiliary energy storage systems. Auxiliary systems obviate the need for excessive electrical currents forced into or drawn from the batteries, as may be required during sudden acceleration or deceleration. Further, due to the reduced energy demand on the batteries in these circumstances, auxiliary energy systems bring with them the potential for significant volume and weight reduction, which translates into higher overall energy system efficiency.
Using a battery pack and an auxiliary energy storage system does not solve all of the problems engendered when such power systems are combined to provide power to an electric drive system, for example. If batteries are used as the primary energy source, and ultracapacitor banks are used for peak-power requirements, size and volume constraints typically prohibit the direct connection of the ultracapacitors to the high-voltage vehicle propulsion bus. Further, it may be prohibitively expensive to size an ultracapacitor bank to match the main bus voltage. Hence, some kind of interface is required between the auxiliary storage device (e.g. ultracapacitors) and the battery-powered bus. While combining multiple energy sources can be achieved in a fundamental fashion (i.e., in theory), there is no known, efficient DC/DC power converter topology which enables such a combination in a practical sense. In addition, in the interests of even higher efficiency, is the desired capability to source and sink current from the energy sources onto the power bus, and from the power bus to the energy sources (i.e., the free exchange of power between multiple sources and a load).
There is also a need to reduce the size of the matching components required when interfacing a primary energy source, such as batteries, and a secondary energy source, such as ultracapacitors. As a practical example, ultracapacitors are currently available in modules of 2,700 F, with a maximum allowed working voltage of 2.3 VDC. Considering the typical electric drive motor requirement of approximately 350 VDC, many ultracapacitors are required for a direct match to the power bus. The typical approach to solving this problem includes the use of a DC/DC converter. However, even using enough ultracapacitors to provide a 50 VDC capability requires a step-up ratio of approximately 7:1. The resulting converter requires very large inductors to handle the currents, and large step-up voltage ratio. Such a conventional solution is also very inefficient. The duty cycle (without enabling free power transfer between sources) involves heavy use of the ultracapacitor, or other auxiliary energy source within the power converter, as is well known in the art. Therefore, there is a need for an efficient, bi-directional DC/DC power converter topology which allows the combination of multiple energy sources for operation of a single load. Any desired combination of low-peak energy delivery capability with a large energy content power source (e.g. batteries or fuel cells), or high-peak energy delivery capability with a low energy content power source (e.g., ultracapacitors or flywheels) should be accommodated. Further, such a topology should embody a design which obviates the need for large inductance values in the power conversion circuitry so that a lower voltage step-up/step-down ratio can be maintained. Such a desired topology inherently increases system efficiency by introducing smaller passive components and reducing the weight/volume of the overall system. Finally, as noted above, there is a need for such a topology which permits the free exchange of power among the sources and load, to include the generation and regeneration of power in an assisted fashion such that the individual energy sources maintain a reduced duty cycle. The step-up (boost) and step-down (buck) capability of the needed topology should be maintained irrespective of the direction of power flow.